The present invention relates to an optical disc, in particular to an optical disc on which information has been recorded in the form of numerous pits with a high density.
Recently, there has been suggested a DVD (Digital Video Disc) which is capable of recording moving pictures having an information amount of two hours, but which has only a size as large as a CD (Compact Disc).
The suggested DVD is constructed by laminating and bonding together two substrates each having a thickness of 0.6 mm. In such a DVD, the information track pitch has a width of 0.74 .mu.m, a shortest pit has a length of 0.4 .mu.m, a recording surface has an information amount of 4.7 G byte.
In an optical system for reproducing the DVD, there has been used a laser diode producing a laser beam having a wave length of 635 nm or 650 nm, and an objective lens having a NA (Numerical Aperture) of 0.6.
However, with a high density recording medium such as DVD, if the depth of each information pit is set to obtain an optimum modulation degree, there will be a problem that other properties such as jitter (fluctuation) will deviate from an optimum value, hence resulting in a bad reproducibility of a DVD.